24 Depth sounding principles

In its simplest form, the depth sounder is purely a timing and display system that makes use of a transmitter and a receiver to measure the depth of water beneath a vessel. Acoustic energy is transmitted perpendicularly from the transducer to the seabed. Some of the transmitted energy is reflected and will be received by the transducer as an echo. It has been previously stated that the velocity of sound waves in seawater is accepted to be 1500 ms-1. Knowledge of this fact and the ability to...

32Speed measurement using water pressure

When a tube, with an opening at its base, is vertically submerged in water, a pressure, proportional to the depth to which the tube is submerged, will be developed in the tube. If the tube is held stationary the pressure remains constant and is termed 'static' pressure. If the tube is now moved through the water, whilst keeping the depth to which it is submerged constant, a second pressure called 'dynamic' pressure is developed. The total pressure in the tube, called a Pitot tube, is therefore...

164HF high frequency band

This frequency band is widely used for terrestrial global communications. Ground waves continue to be further attenuated as the frequency is increased. At the low end of the band, ground wave ranges of a few hundred kilometres are possible but the predominant mode of propagation is the sky wave. Because ionization of the upper atmosphere is dependent upon the sun's radiation, the return of sky waves from the ionosphere will be sporadic, although predictable. At the lower end of the band, during...

Rs422

The use of RS-232 is universal and popular but it does have its limitations. The use of a single line to carry the signal does make it susceptible to noise. Screening the cable can mitigate external noise but will do nothing to stop internal noise. An improved standard introduced by the EIA is the RS-422, which uses a balanced line interface. A pair of lines (Line A and Line B) are used to carry each signal and data is encoded decoded as a differential voltage between the two lines. See Figure...

611 Glossary

Automatic Identification System. Automatic Navigation and Track-keeping System. A system which automatically keeps a ship along a safe pre-planned track. Admiralty Raster Chart Service. The UKHO proprietary Raster Navigational Chart. The area from which the navigation and control of the ship is managed. The total system required for the performance of bridge functions, including bridge personnel, technical systems, man machine interface and procedures. That part of...

10jus

Figure 4.8 Comparison of pulses for Loran-A (lower) and Loran-C (upper). Figure 4.9 Zero crossing times and labels for half-cycles. This figure shows a 'positive' pulse. For a 'negative' pulse the polarity changes but the labels remain the same. Figure 4.9 Zero crossing times and labels for half-cycles. This figure shows a 'positive' pulse. For a 'negative' pulse the polarity changes but the labels remain the same. Each station transmits a pulse which increases rapidly in amplitude and decays...

Set and drift

The VMS-VT sorts the data by type and provides a separate source window for each type of data. To display the source window for a particular data type requires the operator to select the appropriate button on the main menu. Each source window lists a group of sensors appropriate to the data type. The present data from each sensor is included in the window so that the best source can be selected from the list. As an example the position source window, as shown in Figure 6.13, is displayed by...

181General signal fading

In a global system, fading may occur because of the continually changing attenuation factor of an ionospheric layer. Ultraviolet radiation from the sun is never constant, and consequently, the intensity of the ionization of a layer will continually change. The signal attenuation of a specific layer may cause complete signal fade-out as the intensity of the sun's radiation changes. With the exception of this extreme case, the use of automatic gain control (AGC) circuits in a receiver effectively...

814 Glossary

Compass repeaters Controlled gyroscope Drift In the case of a gyrowheel, this is the product of its linear momentum and the radius of the rotor. Containers of viscous liquid to add damping to a gyrocompass. Built-in test equipment. Automatic or manually commanding equipment test circuits. Remote display of compass information. One in which the movement caused by earth rotation is controlled. The apparent movement in azimuth of a gyroscope due to earth rotation. Errors caused by the angular...

111 Summary

Radio waves travel through free space at approximately 300 x 106ms-1. The frequency, wavelength and velocity of the radiowave are interrelated. The radio frequency spectrum is regulated by the International Telecommunications Union (ITU). The Table of Frequency Allocations and radio regulatory documents are revised at the World Administrative Conference (WARC). The radio frequency spectrum is divided into several bands they are VLF, LF, MF, HF, VHF, UHF, SHF and EHF. A propagated radio wave...

251 Description

When chart recording has been selected, transmission is initiated by a pulse from a proximity detector which triggers the chart pulse generator circuit introducing a slight delay, pre-set on each range, to ensure that transmission occurs at the instant the stylus marks zero on the recording paper. This system trigger pulse or that from the trigger pulse generator circuit when the chart is switched off, has three functions to initiate the pulse timing circuit to operate the blanking pulse...

15 Radio frequency spectrum

Table 1.1 indicates how the available frequency spectrum has been divided into usable bands. By referring to this table it is possible to gain some initial idea of the approximate range over which radio waves may be received. For instance, if all other parameters remain constant, the anticipated radio range of signals propagated on the VHF band, or those higher, is effectively that of 'line-of-sight'. Consequently, ship-to-ship communications between a life-raft and a surface vessel could...

531 The space segment

Satellite constellation calls for 24 operational SVs, four in each of six orbital planes, although more satellites are available to ensure the system remains continuously accessible (see Figure 5.5). SVs orbit the earth in near circular orbits at an altitude of 20200 km (10900 nautical miles) and possess an inclination angle of 55 . Based on standard time, each SV has an approximate orbital period of 12 h, but when quoted in the more correct sidereal time, it is 11 h 58 min. Since the earth is...

18 Signal fading

One of the major difficulties encountered when radio waves are propagated via the earth's atmosphere is that of signal fading. Fading is a continual variation of signal amplitude experienced at the antenna input to a receiving system. In practice, fading may be random or periodic but in each case the result will be the same. If the signal input to a receiver falls below the quoted sensitivity figure there may be no output from the demodulator and hence the communications link is broken. If the...

31Introduction

Speed measurement has always been of the utmost importance to the navigator. The accuracy of a dead reckoning position plotted after a long passage without star sights being taken, is dependent upon a sound knowledge of the vessel's heading and speed. To be of value, the speed of any object must be measured relative to some other point. At sea, speed may be measured relative to either the seabed (ground reference speed) or to the water flowing past the hull (water reference speed). Both of...

197 Antenna feed lines

Whilst the connection between the transmitter output and the antenna input appears to be made by a simple wire it is, in fact, made by a balanced transmission line that possesses impedance. Usually, the feed line is a correctly terminated coaxial cable specifically designed for the purpose. For most transmitting and receiving antenna systems the feed line possesses an impedance of 50 or 75 d. Because of its need to handle more power, a transmitter coaxial cable will be physically larger than a...

Rs485

This is also a balanced arrangement similar in detail to RS-422. The RS-485 standard allows up to 32 devices to communicate at half duplex on a single pair of wires, with devices up to 1300 m apart at 120 kbaud, in what is known as a multidrop network. Figure A3.4 shows the arrangement. Figure A3.4 Typical arrangement for an RS-485 two-wire multidrop network. It can be seen from Figure A3.4 that each device has an 'enable' input. Since only one driver stage can be connected to the line at any...

613The International Electrotechnical Commission IEC

Established in 1906, the organization has more than 50 member countries covering 85 of the world's population. Standards established are used in more than 100 countries and there are approximately 200 Technical Committees (TCs) of which TC80 is an important part (see Section 6.3). The IEC collaborates with the ISO in matters of mutual interest and both organizations co-operate on a joint basis with the International Telecommunications Union (ITU). Like the ISO, the IEC is a nongovernmental body...

59 GPS antenna systems

Arguably the antenna is the most critical part of any radiocommunications system but unfortunately it is the piece of hardware that is most often ignored. Carefully designed and constructed, an antenna sits, open to the elements, on board a vessel's superstructure in a position where routine maintenance can be difficult. GPS antennas are small and rigidly constructed and to ensure that they survive the elements they are protected by a raydome. In common with the INMARSAT communications antenna,...

171 Surface wave propagation

The surface wave is a radio wave that is modified by the nature of the terrain over which it travels. This can occasionally lead to difficulty in maritime navigation systems where the wave travels from Figure 1.4 Radio wave modes of propagation. Figure 1.4 Radio wave modes of propagation. one medium to another, over a coastline for instance. The refraction caused in such cases is likely to induce errors into navigation systems. A surface wave will predominate at all radio frequencies up to...

17 Radio wave propagation

Whilst all transmitting antenna systems produce one or more of the three main modes of propagation (see Figure 1.4), one of the modes will predominate. If all other parameters remain constant, the predominant mode of propagation may be equated to the frequency used. For the purpose of this explanation it is assumed that the mode of propagation is dependent upon frequency because that is the only parameter that may be changed by an operator. The three modes of propagation are

222 Salinity pressure and the velocity of the acoustic wave

Since a depth sounder operates by precisely calculating the time taken for a pulse of energy to travel to the ocean floor and return, any variation in the velocity of the acoustic wave from the accepted calibrated speed of 1500 ms-1 will produce an error in the indicated depth. The speed of acoustic waves in seawater varies with temperature, pressure and salinity. Figure 2.2 illustrates the speed variation caused by changes in the salinity of seawater. Ocean water salinity is approximately 3.4...

471 Station selection

Switches S1 and S2 control the two time displays. When the receiver is first initialized (see p. 124) display 2 will be rolling, i.e. displaying various secondary time differences in an ascending sequence. The roll frequency is once every 3 s. When the required secondary time difference appears on display 2, pressing switch 2 will retain that output. If it is required to change the chosen secondary time difference, pressing S2 again will restore the roll action. S1 serves the same function as...

812 Compass repeaters

Remote analogue compass repeaters are simply mechanized compass cards driven either by a stepper motor or a synchro bearing transmission system. Digital heading displays can also be produced by digitizing the stepper 'grey code' waveform before applying it to a suitable decoding system. This section deals with the most popular bearing transmission systems. Figure 8.39 shows a mechanical switching stepper system which, because its robustness, is still found on many merchant ships for bearing...

38 Glossary

ALPHA (Atlas Low Frequency Phased Array) transducer Beamwidth The formation of bubbles on the transducer face causing errors in the system. A flush fitting transducer using multiple elements to create the transmitted beam. The width of the transmitted acoustic pulsed wave. The beam spreads the further it travels away from a transducer. Built-in test circuitry. A self-test or manually operated diagnostic system. Continuous wave transmission. Both the transmitter and receiver are active the whole...

242 Transmission beamwidth

Acoustic energy is radiated vertically downwards from the transducer in the form of a beam of energy. As Figure 2.9 shows the main beam is central to the transducer face and shorter sidelobes are also produced. The beamwidth must not be excessively narrow otherwise echoes may be missed, particularly in heavy weather when the vessel is rolling. A low PRF combined with a fast ship speed Figure 2.9 Transmission beam showing the sidelobes. Figure 2.9 Transmission beam showing the sidelobes. Figure...

114 Glossary

AORE Atlantic Ocean Region East satellite. AORW Atlantic Ocean Region West satellite. DSC Digital selective calling. A NBDP transmission system used for priority alerting. EGC Enhanced group call. A group calling system using Inmarsat-C terminals. EPIRB Emergency position indicating radio beacon. An automatic beacon released from a ship in distress to alert a shore station via the COSPAS SARSAT network of satellites. FEC Forward error correction. An encoding system providing the ability to...

173 Space wave propagation

The space wave, when propagated into the troposphere by an earth surface station, is subject to deflection by variations in the refractive index structure of the air through which it passes. This causes the radio wave to follow the earth's curvature for a short distance beyond the horizon making the radio horizon somewhat longer than the visible horizon. Ship's navigators will know the effect whereby the surface radar range extends slightly beyond the horizon. Space waves propagated upwards...

172 Sky wave propagation

Sky waves are severely influenced by the action of free electrons, called ions, in the upper atmosphere and are caused to be attenuated and refracted, possibly being returned to earth. The prime method of radio wave propagation in the HF band between 3 and 30 MHz is by sky wave. Because under certain conditions, sky waves are refracted from the ionosphere, this band is used extensively for terrestrially-based global communications. Once again, however, there is no clear dividing line between...

56Satellite pass predictions

The system is so well documented and controlled that it has become increasingly easy to predict satellite passes at a given location. Trimble Navigation Limited, one of the biggest manufacturers of GPS equipment, operates a world wide web site that will be of interest to students. It is called GPS Mission Planning and is accessed on http www.trimble.com cgi satview.cgi. It is also interactive and Figure 5.14 Fix accuracy can be improved by selecting appropriate SVs. (a) Two SVs giving a poor...

95 Deadband

Deadband is the manually set bandwidth in which the rudder prime movers do not operate. If the deadband is set too wide, the vessel's course is hardly affected by rudder commands. With the control set narrow, the vessel is subjected to almost continuous rudder action causing excessive drag. Figure 9.9 FU and NFU control of tiller operation. (Reproduced courtesy of Racal Marine Controls.) Figure 9.9 FU and NFU control of tiller operation. (Reproduced courtesy of Racal Marine Controls.) For...

44 LoranC charts

Nautical charts, overprinted with Loran-C LOPs, are available from several sources, including The US National Ocean Survey, for charts principally around the US coast The Defense Mapping Agency, for world-wide charts The Canadian Hydrographic Service, for charts of Canadian waters The UK Hydrographic Office, for charts of British waters. Catalogues of charts and the areas covered are available from the organizations mentioned. The charts are identified in terms of the area covered and the...

71 Introduction

Ever since man first went to sea there has been a requirement for some form of recognition of the seagoing environment to assist in the safe passage to the required destination. Knowledge of the coastline, safe channels for navigation which avoid wrecks, sandbanks etc., and tidal information all play their part in assisting the navigator. Paper charts giving information about particular areas have been around for centuries and hydrographers from various countries have explored the world's...

732 Raster Chart Display System RCDS

This is a system capable of displaying official raster charts that meets the minimum standards required by an appendix to the ECDIS Performance Standard. The raster nautical chart (RNC) is a digital facsimile of the official paper chart and provides a geographically precise, distortion-free image of the paper chart. The IHO proposed a raster chart standard that 'should form a part of the ECDIS performance standards where it would logically fit'. This was approved by the IMO's Maritime Safety...

74 Chart accuracy

Any chart is only as good as the original survey data allows and the accuracy with which that data is recorded on the chart by the cartographer. A navigational chart is referenced to two data horizontal, for latitude and longitude and vertical, for depth and height. Since the beginning of mapmaking, local maps were based on the earth's shape in that area and, since the earth is not a perfect sphere, the shape does vary from location to location. Figure 7.5 shows a representation of a vertical...

57 System errors

Errors in any system arise from a number of sources. They can be predictable or not and avoidable or not. The GPS is no exception. If suffers from error-inducing factors which will downgrade its (UTC> (UTC> Figure 5.15 Trimble mission planning DOP graph taken over 4 hours. A low DOP indicates a high level of accuracy. Figure 5.15 Trimble mission planning DOP graph taken over 4 hours. A low DOP indicates a high level of accuracy. Figure 5.17 Trimble SV sky plot presentation. A GPS receiver...

92Automatic steering principles

Whatever type of system is fitted to a ship, the basic principles of operation remain the same. Before considering the electronic aspects of an automatic steering system it is worthwhile considering some of the problems faced by an automatic steering device. In its simplest form an autopilot compares the course-to-steer data, as set by the helmsman, with the vessel's actual course data derived from a gyro or magnetic repeating compass, and applies rudder correction to compensate for any error...

63 Standards

Those organizations involved in the production of world standards are the International Standards Organization (ISO), the International Electrotechnical Commission (IEC), and the International Telecommunications Union (ITU). The first two organizations work closely together and, as they both have their headquarters in Geneva, some facilities have been amalgamated. The International Maritime Organization (IMO) is responsible for defining the requirements for marine equipment but it does not...

102Radio waves

Radio direction finders work efficiently when using the properties of ground waves or space waves travelling parallel to the earth's surface. Sky waves reflected from the ionosphere seriously affect system accuracy and should be disregarded. A propagated radio wave shown in Figure 10.1 possesses both electrostatic and electromagnetic fields of energy. It is the plane of the electrostatic field that is used to denote the polarization characteristic of the wave. A radio wave possessing a vertical...

101Introduction

With the advent of the GPS and the massive leaps forward in microelectronic technology, the system of radio direction finding (RDF) looks distinctly aged. It is, of course, the oldest of the position fixing systems having been around in one form or another since the First World War. RDF systems used throughout the last century owed their existence to Sir R. A. Watson-Watt who invented the original concept and to Adcock who designed the non-rotating antenna system that eliminated the earlier...

910 Summary

A simple autopilot compares the course-to-steer data, as set by a helmsman, with the vessel's actual course, derived from a master compass, and applies rudder correction to compensate for any error existing between the two input signals. To be effective an autopilot requires the following inputs information about the positional data from the course line, rate of change of course data, and data specifying the cumulative build-up of error. PID control systems use proportional, integral and...

99 Glossary

Counter rudder control Deadband Derivative control Follow-up mode (FU) Integral control Permanent helm Phantom rudder PID controller Proportional control Rudder limit control Virtual ship One in which all the control signals are adapted to suit the vessel and environmental requirements. Determines the degree of opposite helm control which may be applied. A manually set bandwidth in which the rudder movers do not operate. A control signal proportional to the rate of change of the vessel's...

663Specific requirements for different types of bridge equipment

Ships requesting class notation W1-OC shall comply with specific requirements for the following systems course information system (speed and latitude correction) steering systems (manual override control and rate of turn display) speed measuring system (if bottom track then up to 200 m depth) radar systems (two floating EBLs, interswitch, ship track monitoring) traffic surveillance systems (ARPA with two guard zones) position fixing systems (performance standards) watch monitoring and alarm...

Nmea 0183

This NMEA standard specifies the signal parameters, data communication protocol and timing together with sentence formats for serial data bus transmission rates of 4800 baud. The serial data communication between equipments is unidirectional with one 'talker' and possibly many 'listeners'. The data uses ASCII format and typically a message might contain between 11 and 79 characters in length and require transmission at a rate no greater than once every second. The arrangement for...

13 Radio wave radiation

The propagation of radio waves is a highly complex natural phenomenon. It is simplified in the following pages to provide an understanding of the subject with a level of knowledge necessary to comprehend modern navigation systems. Energy is contained in a transmitted radio wave in two forms, electrostatic energy and electromagnetic energy. The radiation of energy from a simple antenna may be described by considering a centre-fed dipole antenna, which is shown electrically in Figure 1.1. Figure...

94 Manual operator controls 941 Permanent helm

This control is intended for use when the vessel is being driven unilaterally off-course by a crosswind. Its function is to apply sufficient permanent rudder angle to offset the drift caused by the wind, thus holding the vessel on the required heading. Permanent helm is also applied automatically when the steering system is in the automatic mode of operation. Automatic application of permanent helm makes no use of the permanent helm control. The degree of rudder offset required for course...

731 Electronic Chart Display and Information System ECDIS

There are several types of electronic chart systems available but only one performance standard has been approved by the International Maritime Organization (IMO) in November 1995. The IMO resolution A817(19) states that the ECDIS should 'assist the mariner in route planning and route monitoring and, if required, display additional navigation-related information'. The system approved is known as the Electronic Chart Display and Information System (ECDIS) and applies to vessels governed by...

472 Normal operation

The chain selector should be set for the chain of the area in which the vessel is operating. Next the function switch should be set to SEL and the notch filters detuned by setting two of them completely clockwise and two completely anticlockwise. The dimmer switch should also be set fully clockwise. The power switch should then be turned ON and after about 4 s both displays should sequentially indicate all secondaries acquired. When the required time difference appears on the display, the...

518 Revision questions

1 What are the basic principles of Kepler's laws of astrophysics 2 How are the orbital period and the velocity of a space vehicle (SV) related 3 How many SVs are used in a full GPS constellation and how many are there in each orbital plane 4 What are the GPS transmission frequencies 5 Why do Navstar SVs transmit on two frequencies 6 How long does it take an SV to transmit an entire navigation data message of 25 frames 7 The GPS uses two codes, the P code and the C A code, for encryption...

532 The control segment

The GPS is controlled from Schriever Air Force Base (formerly Falcon AFB) in Colorado. It is from there that the SV telemetry and upload functions are commanded. There are five monitor stations (see Figure 5.6), which are situated in the Hawaii Islands in the Pacific Ocean, on Ascension Island in the Atlantic, on Diego Garcia in the Indian Ocean, on Kwajalein Island, again in the Pacific, and at Colorado Springs on mainland US territory. SV orbital parameters are constantly monitored by one or...

Acknowledgements

A book of this complexity containing leading edge technology must inevitably owe much to the cooperation of various individuals, equipment manufacturers and organizations. To single out one or more organizations is perhaps invidious. In many cases we have had no personal contact with individuals but despite this they gave freely of their time when information was requested. We are extremely grateful for the assistance that the following companies and organizations gave during the writing of...

52 Basic satellite theory

Whilst it is not essential to understand space technology, it is helpful to consider a few of the basic parameters relating to satellite orbits and the specific terminology used when describing them. A satellite is placed in a pre-determined orbit, either in the nose of an expendable launch vehicle or as part of the payload of a space shuttle flight. Either way, once the 'bird' has been delivered into the correct plane, called the 'inclination', that is the angle formed between the eastern end...

510 GPS receiver designation

Because GPS is freely available to all users throughout the world, the range of available user equipment is vast. There are thousands of manufacturers producing a bewildering range of fixed and mobile equipment, all of which must comply with GPS standards. GPS receiver architecture varies depending upon how it is to be used. The following list itemizes the most popular GPS receiver systems currently produced. The more commonly found commercial receivers are listed first. Amongst the cheapest...

810 A bottomheavy control gyrocompass

Modern bottom-heavy controlled gyrocompasses tend to be sealed gyroscopic units with full computer control and electronic interfacing. For the purpose of system description, this early gyrocompass is a good example of bottom-heavy control used to settle and stabilize a compass. The gyroscopic element, called the sensitive element, is contained within a pair of thin walled aluminium hemispheres joined as shown in Figure 8.34, to form the 'gyroball'. At the heart of this ball is a three-phase...

221 Attenuation and choice of frequency

The frequency of the acoustic energy transmitted in a sonar system is of prime importance. To achieve a narrow directive beam of energy, the radiating transducer is normally large in relation to the wavelength of the signal. Therefore, in order to produce a reasonably sized transducer emitting a narrow beam, a high transmission frequency needs to be used. The high frequency will also improve the signal-to-noise ratio in the system because ambient noise occurs at the lower end of the frequency...

27 A microcomputer echo sounding system

As you would expect, the use of computing technology has eliminated much of the basic circuitry and in most cases the mechanical paper display system of modern echo sounders. Current systems are much more versatile than their predecessors. The use of a computer enables precise control and processing of the echo sounding signal. Circuitry has now reached the point where it is virtually all contained on a few chips. However, the most obvious changes that users will be aware of in modern systems...

25 A generic echo sounding system

Compared with other systems, echo sounder circuitry is relatively simple. Most manufacturers of deep sounding systems now opt for microprocessor control and digital displays, but it was not always so. Many mariners preferred the paper-recording echo sounder because the display was clear, easy to read and provided a history of soundings. Marconi Marine's 'Seahorse' echo sounder (Figure 2.11) was typical of the standard paper-recording echo sounder. Built in the period before microprocessor...

87Compass errors

Sperry Navigation Systems

The accuracy of a gyrocompass is of paramount importance, particularly under manoeuvring situations where the compass is interfaced with collision-avoidance radar. An error, either existing or produced, between the actual compass reading and that presented to the radar could produce potentially catastrophic results. Assuming that the compass has been correctly installed and aligned, the static compass errors briefly listed below, should have been eliminated. They are, however, worthy of a brief...

76 Automatic Identification System AIS

Automatic Identification System (AIS) is a shipborne transponder system capable of broadcasting continuously, using the VHF marine band, information about the ship. Such information could include ship identification data, i.e. ship name, call sign, length, breadth, draught etc. type of cargo carried and whether it was hazardous in nature course and manoeuvring data position to GPS accuracy limits. Such broadcast information would be capable of reception by other AIS-equipped ships and by shore...

105 Errors

Although RDF systems are subject to errors, caused mainly by environmental effects, if a fixed loop or Adcock RDF system is correctly installed and accurately calibrated the errors can be reduced to virtually zero. As with any electronic system, it is important to appreciate the error causes and cures. The major error factors affecting RDF systems installed on merchant ships are listed below. Some of these have minimal effect at VHF but they have been included here for reference. This error is...

Navigation

In each case the tasks that have to be performed are specified and the siting of relevant instruments equipment required for those tasks is defined. As an example, the workstation for navigation is specified to enable the following tasks to be performed determine and plot the ship's position, course, track and speed effect internal and external communications related to navigation monitor time, course, speed and track, propeller revolutions, pitch indicator and rudder angle. The following...

231 Electrostrictive transducers

Electrostrictive Transducer

Certain materials, such as Rochelle salt and quartz, exhibit pressure electric effects when they are subjected to mechanical stress. This phenomenon is particularly outstanding in the element lead zirconate titanate, a material widely used for the construction of the sensitive element in modern electrostrictive transducers. Such a material is termed ferro-electric because of its similarity to ferromagnetic materials. The ceramic material contains random electric domains which when subjected to...

513 GPS on the web

GPS enjoys massive coverage on the world wide web and there are simply far too many sites to list here. However, some of the better sites are worth a visit and are listed below. An essential site for all navigators. United States Coast Guard site with numerous pages of data on GPS, Loran-C and US coastal navigation notices. Massive amounts of detail about GPS time transfer, current constellation status and health. GPS and GLONASS alphabetical index link site to dozens of other relevant sites....

58 Differential Gps Dgps

As has already been stated, the accuracy of GPS fixes can be vastly improved using differential techniques. Experimental differential systems have been in use for some years as part of earlier hyperbolic earth-based navigation systems. DGPS is merely an improvement of those now outdated systems. The principle, as shown in Figure 5.18, is that GPS data from SVs are downloaded to both a mobile station and a fixed station at a precise location. A computer at the fixed site calculates the...

62 Design criteria

In the 1960s Planned Ships Bridges were available from at least one manufacturer and fitted on some vessels. This was probably the first attempt to construct a bridge within design concepts that took into consideration the operational requirements of the vessel. Integrated navigation systems and integrated bridge systems have evolved from those days and the concept is now accepted, with a variety of systems available from many different manufacturers. Certain classification societies have...

34 Speed measurement using acoustic correlation techniques

Unlike the previously described speed log, which measure the vessel's speed with respect to water only, the SAL-ICCOR log measures the speed with respect to the seabed or to a suspended water mass. The log derives the vessel's speed by the use of signal acoustic correlation. Simply, this is a way of combining the properties of sonic waves in seawater with a correlation technique. Speed measurement is achieved by bottom-tracking to a maximum depth of 200 m. If the bottom echo becomes weak or...

321 A pressure tube speed logging system

Graph Pitot Tube

Figure 3.3 shows a typical installation of the Pitot system on board a vessel with a double bottom. The Pitot tube is encased in a sea-cock arrangement with valve control, to enable the tube to be withdrawn, without shipping water, when the vessel goes alongside. The static pressure opening is controlled by the use of a valve. Both dynamic and static pressures are transferred via air collectors and strainer valves to the pressure chamber. The strainer valves are designed to prevent water...

45 Position fixing using the LoranC System

For a particular location covered by more than one Loran-C chain, the operator should select the best chain available, and where possible, select a chain that can be used throughout the voyage so that the receiver can 'lock on' to the signal and 'track' throughout the trip. Having selected a chain, it is necessary to select secondary stations which give the best fix. There may be a choice of more than two master-secondary station pairs and it is essential to choose those two pairs which give...

75 Updating electronic charts

As mentioned on page 228 with reference to the UKHO's ARCS system, updates for all charts affected by Notice to Mariners (up to about 200 a week) are generated, checked and placed on a weekly ARCS Update CD-ROM which includes temporary and preliminary notices. This provides error-free automatic corrections and provides cumulative updates with only the latest update CD-ROM required. The CD-ROMs are sent to chart agents who then send them to shipping companies as required. NOAA provides...

813 The magnetic repeating compass

Cross Section Fluxgate Compass

Magnetic compasses are still popular with mariners and can easily be converted into a repeating compass with the addition of a flux gate assembly. A flux gate element is effectively a magnetometer that is used to detect both the magnitude and the direction of a magnetic field. Flux gate elements in common use are of the 'second harmonic' type, so called because if excited by a fundamental frequency, f, an output voltage will be generated which varies in both phase and amplitude, depending upon...

93 A basic autopilot system

Basic Autopilot System

The simplest form of autopilot is that shown in Figure 9.6. An output from a gyro or magnetic repeating compass is coupled to a differential amplifier along with a signal derived from a manual course-setting control. If no difference exists between the two signals, no output will be produced by the amplifier and no movement of the rudder occurs. When a difference is detected between the two sources of data, an output error signal, proportional in magnitude to the size of the difference, is...

Preface

This new edition of Electronic Navigation Systems has been extensively rewritten to provide navigators with a detailed manual covering the principles and applications of modern systems. The past decade has been witness to huge advances in technology and no more so than in maritime navigation and position fixing. As you might expect, spearheading this technological advance has been the computer. It has become as common on board ships as in our normal lives where it now influences virtually...

106 RDF receiving equipment

In the early days of radio direction finding, receivers were almost always manually operated. Today however, all RDF equipment is automatic. The first automatic receivers depended upon the use of a servomotor to physically drive the RDF compass card to indicate the relative bearing. 10.6.1 An automatic system using a servomotor This type of RDF has at its heart a low power two-phase servo that, via a mechanical drive mechanism, rotates the goniometer search coil and bearing pointer. This type...

5122 Garmin GPS receiver specifications

Global Gps Coverage

Amongst a range of GPS equipment designed for the maritime market, Garmin offers a 12-channel GPS receiver (with an optional DGPS receiver) combined with a navigation plotter. This versatile equipment, known as the GPSMAP 225, is representative of the way that system integration is making life easier for the maritime navigator. The GPSMAP 225 effectively presents an electronic charting navigation system based on a 16-colour active-matrix TFT display that modern navigators will feel comfortable...

96 Phantom rudder

Dependent upon the setting of the 'phantom rudder speed' control, a determinate d.c. voltage is applied to an integrator input resistance with the result that the circuit starts to generate the positive going ramp voltage Vp defined by the solid line in Figure 9.11. It should be noted that the polarity of the integrator output is the reverse of that of the translator output Vt, hence the provision of separate voltage scales on the y-axis of the graph. It is arranged so that the slope of Vp and...

6101 Voyager by Furuno Electric Co

An automatic navigation system designed by Furuno to meet the requirements for one-man bridge operation and the new ECDIS standards is the Voyager Integrated Bridge System. The system was designed to meet the class notation W1-OC of DNV, Norway. The system is modular which allows it to be set up to meet the requirements of the user and to provide capability for future expansion of the system as necessary. The complete system requirement comes from a single supplier with the claimed benefits of

33 Speed measurement using electromagnetic induction

Electromagnetic Log

Electromagnetic speed logs continue to be popular for measuring the movement of a vessel through water. This type of log uses Michael Faraday's well-documented principle of measuring the flow of a fluid past a sensor by means of electromagnetic induction. The operation relies upon the principle that any conductor which is moved across a magnetic field will have induced into it a small electromotive force (e.m.f.). Alternatively, the e.m.f. will also be induced if the conductor remains...

84 The northseeking gyro

The gyrospin axis can be made meridian-seeking (maintaining the spin axis parallel to the earth's spin axis) by the use of a pendulum acting under the influence of earth gravity. The pendulum causes a force to act upon the gyro assembly causing it to precess. Precession, the second fundamental property of a gyroscope, enables the instrument to become north-seeking. As the pendulum swings towards the centre of gravity, a downward force is applied to the wheel axle, which causes horizontal...

85 A practical gyrocompass

Gyrocompas Service Manual

The apparent tilting of the gyroscope can be reduced by producing an offset controlling force, which in effect creates 'anti-tilt' precession allowing the unit to settle in the meridian. This is achieved by creating a force about the vertical axis to cause precession about the horizontal axis. This is achieved, in this gyro system, by offsetting the mercury ballistic controlling force slightly to the east of the vertical. The point of offset attachment must be precise so that damping action...

36 Principles of speed measurement using the Doppler effect

Doppler Effect Ship

The phenomenon of Doppler frequency shift is often used to measure the speed of a moving object carrying a transmitter. Modern speed logs use this principle to measure the vessel's speed, with respect to the seabed, with an accuracy approaching 0.1 . If a sonar beam is transmitted ahead of a vessel, the reflected energy wave will have suffered a frequency shift (see Figure 3.16), the amount of which depends upon the velocity of the sonar energy wave the velocity of the transmitter (the ship)....

234 Transducer siting

The decision of where to mount the transducer must not be made in haste. It is vital that the active face of the transducer is in contact with the water. The unit should also be mounted well away from areas close to turbulence that will cause noise. Areas close to propellers or water outlets must be avoided. Aeration is undoubtedly the biggest problem encountered when transducers are wrongly installed. Air bubbles in the water, for whatever reason, will pass close to the transducer face and act...

112 Revision questions

1 Why does it appear that the radiocommunications range on MF HF is greater at night than during the day at your location 2 How is it possible to receive LF radio waves in regions that are radio-shadow areas to VHF radio 3 Unwanted sky wave reception gives rise to errors in some navigation systems, typically Loran-C. Why is the effect more prevalent at night 4 How may frequency selective fading be minimized in a receiver system 5 How are the receptive properties and an antenna's physical length...

104 A fixed loop antenna system

At the heart of this system are two permanently fixed loop antennae, mounted on the same mast or base at 90 to each other, one on the fore-and-aft line and the other on the port-and-starboard line of a vessel. An early manual RDF input system is shown in Figure 10.6 to illustrate the principle. In this case each precisely mounted loop antenna is connected to a pair of precisely aligned fixed coils in a goniometer, a tiny transformer arrangement recreating the electromagnetic fields of the loop...

816 Revision questions

1 Describe what you understand by the term gyroscopic inertia 2 What do you understand by the term precession when applied to a gyrocompass 3 Why is a free gyroscope of no use for navigation purposes 4 How is earth's gravity used to turn a controlled gyroscope into a north-seeking gyroscope 5 How is a north-seeking gyroscope made to settle on the meridian and indicate north 6 When first switched on a gyrocompass has a long settling period, in some cases approaching 75 min. Why is this 7 Explain...

97 The adaptive autopilot

Autopilot systems so far described have operated under various command functions, the origins of which have been small signals produced by feedback loops. The rudder command-loop signals have been further modified by the proportional, integral and derivative terms to form the nucleus of the PID autopilot systems. The adjustment of operator controls on the PID autopilot requires considerable expertise if the system is to operate efficiently. It is not feasible to continually reset Figure 9.12...

113 The Navtex system

Navtex Coverage

NAVTEX is not a position fixing system, it is an information network. The service forms an integral part of both the Global Maritime Distress and Safety System (GMDSS) and the World Wide Navigational Warning Service (WWNWS) operated by the International Maritime Organization (IMO). These broadcast systems are designed to provide the navigator with up-to-date navigational warnings in English and, using the EGC SafetyNET message service, provide a means of shore-to-ship alerting announcing...

112 The system

Basic Concept Cospas Sarsat System

After a lengthy implementation period, the GMDSS became fully operational on 1 February 1999. The basic concept, shown in Figure 11.1, shows that a ship in distress is effectively inside a highly efficient radio net. If the casualty is correctly fitted with GMDSS equipment it will be in a position to alert and communicate with a wide range of ship- and shore-based radio stations and through them initiate a coordinated SAR operation based on a rescue co-ordination centre (RCC). GMDSS relies...

911 Revision questions

1 What are the three main control functions known as PID in an autopilot system 2 What are the three main feedback parameters required by an autopilot system 3 What parameter determines the extent of proportional control fed back into the system 4 Why is the rate of change of the ship's deviation from its course important in autopilot 5 What is an error summing system 6 What is the function of the weather control 7 The permanent helm control provides a degree of bias in the system. Why is this...

5121 Trimble GPS receiver specifications

At the top of the Trimble's GPS range is the NT300D, a 12-channel parallel GPS receiver, capable of tracking up to 12 satellites simultaneously and also containing a dual-channel differential beacon receiver. The equipment is capable of submetre accuracy derived from carrier-phase filtered L1 pseudo-range calculations. In addition, vessel velocity is obtainable from differentially corrected Doppler measurements of the L1 carrier. Position information is displayed on a backlit LCD screen in one...

474 Specification of the LC90 MarkII Receiver

Receiver sensitivity Differential dynamic range Interference rejection Tracking capacity Six notch filters, four of which are auto and two are preset. Master and up to a maximum of five secondaries. Tracking speed 80 knots nominal. Nominally 5min, depending on signal conditions. S N, CYC, tracking point, interference frequency and level, notch filter settings etc. Save function (entry of waypoint and event) 100 points (from no.00 (OS position) to no. 99) 20 points (from no.100 to no. 119)....

42 System principles

The loran transmitter stations send out a stream of pulses at a specified rate known as the pulse repetition frequency (PRF) or the pulse repetition rate (PRR). The pulse repetition period is the reciprocal of the PRF. Assume the PRF is 25, i.e. 25 pulses are transmitted every second, then the period of the pulse is 1 25 s or 40000 ps. The pulse width is 40 ps for Loran-A and 250 ps for Loran-C. Assuming that the velocity of radio waves in free space is 3 x 108 ms-1, then the distance travelled...

77 Navmaster Electronic Navigation System

There are a multitude of suppliers of software suitable for implementing an electronic navigation system, requiring only the hardware and suitable electronic charts to produce an ECDIS or an ECDIS in RCDS mode. The 'Navmaster Professional' from PC Maritime of Plymouth, UK is used as a basis for showing how the software can assist the navigator in passage planning, position logging and navigation management, providing as it does a continuous display of vessel positions received from GPS and...

91Introduction

It has already been implied that a modern merchant vessel must be cost-effective in order to survive the ever-increasing pressure of a financially orientated industry. A good automatic pilot, often called an Autohelm, although a registered trade name, can improve the profit margin of a vessel in two ways. First, it enables a reduction to be made in the number of ships' personnel, and second, a considerable saving in fuel can be achieved if the vessel makes good its course with little deviation....

6103 Sperry Marine Voyage Management System Vision Technology VMSVT

The Sperry VMS-VT system, provided by Litton Marine Services, is a computer-based navigation, planning and monitoring system which typically consists of two or more computer workstations connected by a local area network (LAN). A typical arrangement for a VMS-VT system is shown in Figure 6.7. Figure 6.7 Typical arrangement for the Voyage Management System - Vision Technology (VMS-VT). (Reproduced courtesy of Litton Marine Systems.) Figure 6.7 Typical arrangement for the Voyage Management System...

410 Revision questions

1 Explain briefly the concept behind the use of low-frequency pulsed signals transmitted from land-based stations to determine the position of a ship, or aircraft, that carries a receiver suitable for the reception of such signals. 2 A transmitter emits a pulse which is intercepted by a second transmitter 150 km away. If the speed of transmission of the pulse is 3 x 108ms-1, how long does it take the pulse to travel between the stations 3 What would be the time taken in question 2 if the speed...

82 Gyroscopic principles

At the heart of a marine gyrocompass assembly is a modern gyroscope consisting of a perfectly balanced wheel arranged to spin symmetrically at high speed about an axis or axle. The wheel, or rotor, spins about its own axis and, by suspending the mass in a precisely designed gimbals assembly, the unit is free to move in two planes each at right angles to the plane of spin. There are therefore three axes in which the gyroscope is free to move as illustrated in Figure 8.1 In a free gyroscope none...

78 Glossary

Automatic Identification System, see Transponder. Admiralty Raster Chart Service. The UKHO proprietary RNC. Automatic Radar Plotting Aid. The smallest unit for geographical data. Each cell has a unique address in memory and may possess different data volume and size characteristics. A graphical representation of an object or characteristic. Chart Information System. A display where the heading of own ship is upwards on the screen and the chart moves relative to...

98 An adaptive digital steering control system

Stirling Pump Schematic

Sperry Marine Inc., now part of the Litton Marine Systems group, is a traditional manufacturer of compass and control equipment, and their ADG 3000VT Adaptive Digital Gyropilot Steering Control system is a good example of an up-to-date autopilot using many of the principles described in this chapter (see Figures 9.13 and 9.14). At the heart of the autopilot is a sophisticated microcomputer and electronic circuitry providing control signal outputs to the steering gear pump controllers. The...

811 Starting a gyrocompass

As has been previously stated, from start-up a gyrocompass needs time to settle on the meridian. The time taken depends upon the make, model and the geographic location of the compass, but in general it is between one and several hours. The duration also depends upon whether the gyro wheel is already rotating or not. If the compass has been switched off, it will take much longer to bring the compass into use. Inputting the ship's heading to reduce the initial error factor can reduce the time...

89 A digital controlled topheavy gyrocompass system

Topheavy Marina

In common with all other maritime equipment, the traditional gyrocompass is now controlled by a microcomputer. Whilst such a system still relies for its operation on the traditional principles already described, most of the control functions are computer controlled. The Sperry MK 37 VT Digital Gyrocompass (Figure 8.29) is representative of many gyrocompasses available. The system has three main units, the sealed master gyrocompass assembly, the electronics unit and the control panel. The master...